By convention, the power of a microscope is defined as the ratio of the apparent size of an object viewed with the microscope to the apparent size of the same object viewed with the unaided eye at a distance of 250 mm. More particularly the comparison is made with the microscope so focused that the apparent distance to the object as viewed with the microscope is 250 mm as determined by the required focus of the eye.
When viewing an object at a distance of 250 mm, the angle of convergence of the lines of sight from the viewers two eyes is approximately 15.degree.. Accordingly the conventional 3-D stereo microscope has two objective lenses and two eyepieces arranged to view a point from two directions separated by 15.degree..
This arrangement of the objective lens limits their numerical aperture to the sine of 71/2.degree. or 0.13. Objective lenses with higher numerical apertures would simply interfere with each other when set at a 15.degree. angle to each other. The resolution and hence the maximum useful power of a microscope is proportional to the numerical aperture of the objective lens. Conventional objective lenses with numerical apertures as high as 0.95 are commonly used for high power microscopes and oil immersion objective lenses with numerical apertures as high as 1.30 are available as well. As a result, there are in common use, microscopes ten times as powerful as the most powerful useful stereo microscopes of conventional design.
In addition to the limitation of numerical aperture characteristic of stereo microscopes, the depth of focus of the high numerical aperture objectives of the higher power microscope is so limited that the 3-D stereo effect would be useless.
Many better high power microscopes are fitted for binocular viewing. These microscopes have a beamsplitter, various mirrors, and two eyepieces arranged so that both eyes are used to view an object. In these microscopes, however, both eyes view identical images so that there is no 3-D stereo effect.